Dimension reference gauges are used in testing measuring devices such as coordinate measuring machines.
Step gauges corresponding to a plurality of lengths as well as various gauge blocks whose end-to-end dimension is highly accurately calibrated are used for the dimension reference gauge.
The step gauges are comb-shaped components having alternately arranged protrusions and recesses, where a plurality of reference dimensions are defined between end faces of the protrusions. The step gauges are produced by alternately arranging measurement blocks defining the protrusions and spacer blocks defining the recesses, and fixing the arranged blocks on a holder. Alternatively, the step gauges are produced by cutting a single component into a form of the comb-shaped component.
A calibration value of the end-to-end dimension of the step gauges is defined as a length at a specific temperature and is often defined as a length at 20 degrees C. (industrial standard temperature).
In testing a coordinate measuring machine, the measured length has to be converted to a length at a temperature for the calibration. Such conversion is usually referred to as a length temperature correction. At this time, it is necessary that a coefficient of thermal expansion (CTE) of the step gauge is accurately known.
The CTE used for the temperature correction is written in a calibration certificate or a test certificate for most of the dimension reference gauges including step gauges. Such a CTE is indicated with tolerance.
When a step gauge is used for testing a coordinate measuring machine, the tolerance is considered as a factor of uncertainty in determining uncertainty of the test. Accordingly, it is required that the CTE of the step gauge is highly accurately evaluated in order to reduce the uncertainty in the test.
The CTE of an object including a dimension reference gauge is obtained by changing the temperature of the object and measuring a length variation of the object due to the temperature change.
Specifically, a CTE α is given by a formula α=(ΔL/L)·(1/ΔT), where ΔT=T−To (T: a current temperature, To: a reference temperature) represents the temperature variation, and ΔL=L−Lo (L: a length of the object at the current temperature T, Lo: a length of the object at the reference temperature To) represents the length variation (thermal expansion).
In a dimension reference gauge such as a step gauge, the length L of the object is more than 105 times larger than the length variation ΔL. Accordingly, the accuracy of the value of the length L has relatively a small impact on the value of the CTE α.
Accordingly, in order to highly accurately calculate the CTE α, it is necessary to highly accurately measure the temperature variation ΔT and the length variation ΔL.
In order to measure the CTE α of an object, a measurement method using an optical interferometer has been proposed (see Patent Literature 1: JP 3897655 B).
In Patent Literature 1, two pairs of optical interferometers opposed on a common measurement axis are used to highly accurately measure an end-to-end dimension of a measurement target (e.g. gauge block). Then, the measurement target is directly heated with a soaking plate brought into contact with the measurement target, and measured in length at different temperatures. A thus-obtained thermal expansion due to temperature change is used to calculate the CTE.
However, such a CTE measurement method uses the soaking plate to heat the measurement target. Thus, only a surface of the measurement target in contact with the soaking plate is locally heated, causing an uneven temperature distribution of the measurement target. Consequently, a thermal expansion in the measurement target fails to be even to cause an error in the measurement of the CTE.
In order to address such a problem, a method using a temperature-controlled chamber for controlling a temperature of a step gauge has been proposed (see Patent Literature 2: JP 2004-226369 A).
In the method disclosed in Patent Literature 2, the step gauge (measurement target) is placed in the temperature-controlled chamber. Further, a probe of an external coordinate measuring machine is introduced through an opening of the temperature-controlled chamber and the length of the step gauge is measured using the probe. A temperature setpoint of the inside of the temperature-controlled chamber is changed to measure the length at different temperatures. A thus-obtained thermal expansion due to temperature change is used to calculate a CTE.
In such a measurement method, the temperature of a gas inside the temperature-controlled chamber is regulated to a constant value, and the temperature of the step gauge is indirectly changed with the gas. The temperature of the step gauge can thus become even, allowing for a highly accurate CTE measurement.
However, in the method of Patent Literature 2, the temperature of the step gauge is gradually changed because the temperature is indirectly changed via the gas. Such a method requires a long time to stabilize the temperature of the step gauge at the temperature setpoint, and is thus unlikely to achieve an efficient measurement.